French patent no. 2,963,152 describes a magnetic memory element as shown schematically in FIGS. 1A, 1B and 10. FIGS. 1A and 1B respectively show a sectional view and a perspective view of a magnetic memory element as described in connection with FIGS. 1c-1f, 2a-2b and 3a-3d of French patent no. 2,963,152. FIG. 10 is a simplified top view of this memory element.
As illustrated by FIGS. 1A and 1B, this memory element comprises a contact 3 above a conductive track 1. The contact 3 comprises a stack of regions, each of which is formed by a portion of a thin layer or a stack of several thin layers. The conductive track 1 is for example formed on a substrate 5 made up of a silicon wafer covered with a layer of silicon oxide and is connected across terminals A and B. The stack making up the contact 3 successively comprises, from the track 1, a region 10 made from a nonmagnetic conductive material, a region 11 made from a magnetic material, a region 12 made from a nonmagnetic material, a region 13 made from a magnetic material and an electrode 14. The material of the layer 12 can be conductive; this is preferably an insulating material thin enough to be able to be traversed by tunnel effect electrons. There is a structural difference between the nonmagnetic regions 10 and 12 so as to have an asymmetrical system in a direction orthogonal to the plane of the layers. This difference may in particular result from the difference in material, thickness, or growth mode of these layers.
Lists of materials able to make up the various layers are given in the aforementioned patents. The magnetic materials of the regions 11 and 13 are formed under conditions such that they have a magnetization oriented orthogonally to the plane of the layers. The magnetic material of the layer 13 is formed under conditions such that it retains an intangible magnetization (trapped layer). The upper electrode layer 14 is connected to a terminal C.
The programming of the memory element is done by circulating a current across the terminals A and B, while a field H oriented horizontally (parallel to the plane of the layers in the direction of the current across the terminals A and B) is applied. Depending on the relative directions of the current across the terminals A and B and the field vector H, the layer 11 is programmed such that its magnetization is oriented upward or downward.
To read this memory element, a voltage is applied between the terminal C and one or the other of the terminals A and B. The resulting current between the terminal C and one or the other of the terminals A and B assumes different values depending on the relative direction of the magnetizations of the layers 11 and 13: high value if the two magnetizations are in the same direction and low value if the two magnetizations are in opposite directions.
One characteristic of the memory element described above is that its programming is done owing to a current circulating across the terminals A and B and magnetic field applied in the plane of the layers, parallel to the current. No current circulates from the terminal A or B toward the terminal C during programming. This has the advantage of completely separating the read and write operations of the memory element.
Many alternative embodiments are possible. In particular, each layer previously described can be made up of a stack of layers in a manner known in the art to acquire the desired characteristics.
The layer portion 10 made from a nonmagnetic conductive material can be omitted, as long as the track 1 is made from a nonmagnetic material suitable for the growth of the magnetic layer 11. The track 1 may then have an excess thickness below the contact 3. For the reversal of the magnetization in the layer 11 to be possible, it is also necessary for spin-orbit pairs to be present in the magnetic layer. To that end, it is for example necessary for the layer in contact with this layer 11 (or separated from it by a fine separating layer) to be made up of a material or compound of materials with strong spin-orbit coupling. Another solution is, for example, that the contact between the magnetic layer 11 and one or the other of the layers 10 and 12 creates this spin-orbit coupling; this may for example happen through hybridization of the magnetic layer 11 with the layer 12 if the latter is made up of an insulator (see “Spin-orbit coupling effect by minority interface resonance states in single-crystal magnetic tunnel junctions”, Y. Lu et al. Physical Review B, Vol. 86, p. 184420 (2012)).
It will be noted that the memory element of FIGS. 1A and 1B can be broken down into two elements: a storage element comprising the track 1 provided with terminals A and B and the layer portions 10, 11 and 12, and a read element comprising, in the example given above, the layers 13 and 14 and the electrode C. With the same storage element, various read modes could be considered, for example optical reading.
FIG. 10 is a simplified top view of the contact 3. Only the track 1 and the contact 3 are shown, as well as the terminals A and B connected to contacts 15 and 16.
As previously indicated, the memory element of FIGS. 1A to 10 is programmable by applying a current across the terminals A and B simultaneously with the application of a magnetic field having a nonzero component in the direction of the current. Examples of means for generating a magnetic field are given in the aforementioned patent application. The application of an outside field or the production of specific magnetic layers capable of creating the field H raises practical production problems.
Patent application US 2014/0010004 describes a magnetic memory element that can be programmed by applying a current in the absence of a magnetic field. FIG. 2 is a schematic bottom view of a magnetic memory element corresponding to FIG. 18A of this patent application. A magnetic contact 20 comprises a stack of layer portions similar to the layers of the magnetic contact 3 described in connection with FIGS. 1A to 10. The contact 20 is in the form of an elongated rectangle. Two different electrodes 24A and 24B positioned at the ends of the rectangle and protruding from a large side of the rectangle are connected to terminals A and B and make it possible to circulate a current in the magnetic layer 11. The direction of the flow of the current, from the terminal A toward the terminal B or from the terminal B toward the terminal A defines the programmed value. Such a configuration of the memory element comprising separate electrodes below the contact poses various production problems.
There is a need for a memory element programmable by applying a current in the absence of a magnetic field that is easy to produce and sensitive to weak currents.